1 ! Copyright (c) 2009 Jon Harper.
2 ! See http://factorcode.org/license.txt for BSD license.
3 USING: kernel math math.functions project-euler.common ;
6 ! http://projecteuler.net/index.php?section=problems&id=255
11 ! We define the rounded-square-root of a positive integer n as the square root
12 ! of n rounded to the nearest integer.
14 ! The following procedure (essentially Heron's method adapted to integer
15 ! arithmetic) finds the rounded-square-root of n:
17 ! Let d be the number of digits of the number n.
18 ! If d is odd, set x_(0) = 2×10^((d-1)⁄2).
19 ! If d is even, set x_(0) = 7×10^((d-2)⁄2).
21 ! Repeat: [see URL for figure ]
23 ! until x_(k+1) = x_(k).
25 ! As an example, let us find the rounded-square-root of n = 4321.
26 ! n has 4 digits, so x_(0) = 7×10^((4-2)⁄2) = 70.
28 ! [ see URL for figure ]
30 ! Since x_(2) = x_(1), we stop here.
32 ! So, after just two iterations, we have found that the rounded-square-root of
33 ! 4321 is 66 (the actual square root is 65.7343137…).
35 ! The number of iterations required when using this method is surprisingly low.
36 ! For example, we can find the rounded-square-root of a 5-digit integer
37 ! (10,000 ≤ n ≤ 99,999) with an average of 3.2102888889 iterations (the average
38 ! value was rounded to 10 decimal places).
40 ! Using the procedure described above, what is the average number of iterations
41 ! required to find the rounded-square-root of a 14-digit number
42 ! (10^(13) ≤ n < 10^(14))? Give your answer rounded to 10 decimal places.
44 ! Note: The symbols ⌊x⌋ and ⌈x⌉ represent the floor function and ceiling
45 ! function respectively.
52 ! same as produce, but outputs the sum instead of the sequence of results
53 : produce-sum ( id pred quot -- sum )
54 [ 0 ] 2dip [ [ dip swap ] curry ] [ [ dip + ] curry ] bi* while ; inline
57 number-length dup even?
58 [ 2 - 2 / 10 swap ^ 7 * ]
59 [ 1 - 2 / 10 swap ^ 2 * ] if ;
61 : ⌈a/b⌉ ( a b -- ⌈a/b⌉ )
64 : xk+1 ( n xk -- xk+1 )
65 [ ⌈a/b⌉ ] keep + 2 /i ;
67 : next-multiple ( a multiple -- next )
68 [ [ 1 - ] dip /i 1 + ] keep * ;
71 ! Gives the number of iterations when xk+1 has the same value for all a<=i<=n
72 :: (iteration#) ( i xi a b -- # )
74 [ drop i b a - 1 + * ]
75 [ i 1 + swap a b iteration# ] if ;
77 ! Gives the number of iterations in the general case by breaking into intervals
78 ! in which xk+1 is the same.
79 :: iteration# ( i xi a b -- # )
84 ! set up the values for the next iteration
85 [ nip [ 1 + ] [ xi + ] bi ] 2keep
86 ! set up the arguments for (iteration#)
87 [ i xi ] 2dip (iteration#)
89 ! deal with the last numbers
90 [ drop b [ i xi ] 2dip (iteration#) ] dip
93 : (euler255) ( a b -- answer )
95 [ [ drop x0 1 swap ] 2keep iteration# ] 2keep
100 : euler255 ( -- answer )
101 13 14 (euler255) 10 nth-place ;
103 ! [ euler255 ] gc time
104 ! Running time: 37.468911341 seconds